1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material, its method of manufacture and a thermoelectric module using this, and, in addition, to a method of hot forging.
2. Description of the Related Art
Application of electronic cooling elements utilizing the Peltier effect or Ettinghausen effect, or thermoelectric power generating elements utilizing the Seebeck effect over a wide range is noted on account of their simple construction, ease of handling, and ability to maintain stable characteristics. In particular regarding electronic cooling elements, research is being conducted in many places aimed at temperature stabilization etc. of optoelectronics or semiconductor lasers etc., on account of the capability that they possess for precise temperature control of local cooling and in the vicinity of room temperature.
As shown in FIG. 12, a thermoelectric module employed in such electronic cooling and thermoelectric power generation is constructed such that a pn element pair is formed by joining a p-type semiconductor 5 and n-type semiconductor 6 through a metallic electrode 7, a plurality of such pn elements being arranged in series, heat being generated at one end while cooling occurs at the other end, depending on the direction of the current flowing through the junctions. A material of large figure of merit Z(=.alpha..sup.2 /.rho.K) expressed by the Seebeck coefficient .alpha., resistivity .rho.and thermal conductivity K, which are constants characteristic of the material, is employed as the material of such a thermoelectric element.
Most thermoelectric semiconductor materials have anisotropy of thermoelectric performance depending on their crystal structure i.e. the figure of merit Z is different depending on crystal orientation. A single crystal material is therefore employed with current being passed in a crystal orientation of large thermoelectric performance. In general, anisotropic crystals are subject to cleavage and are brittle, so, as a practical material, instead of employing single crystals, a material is employed in which alignment is effected in a crystal orientation of large thermoelectric performance by unidirectional solidification achieved by the Bridgman method etc.
However, albeit not to the same degree as a single crystal, such a unidirectional solidified material is still brittle, and problems are experienced during element working regarding cracking and/or chipping of the elements. In contrast to such crystalline material, powder sintered material has no cleavage and the material strength is enormously better, but the alignments of the crystal orientations are random or, if there is crystal alignment, this shows a gently sloping distribution, so there was the problem that the thermoelectric performance was inferior to that of crystalline materials. Thermoelectric semiconductor materials having both satisfactory strength and performance were thus hitherto unavailable. Specifically, the crystalline materials that were typically employed as electronic cooling elements were mixed crystals of bismuth telluride (Bi.sub.2 Te.sub.3), antimony telluride (Sb.sub.2 Te.sub.3) and bismuth selenide (Bi.sub.2 Se.sub.3), but these crystals have the problems of being subject to severe cleavage and that the yield was very considerably lowered owing to cracking and chipping on undergoing slicing and dicing steps etc. to obtain the thermoelectric element from the ingot.
Formation of powder sintered elements has therefore been tried in order to improve mechanical strength. When the material is employed in the form of a powder sintered body instead of crystals, the problem of cleavage is eliminated, but, as mentioned above, performance is inferior due to the poor alignment. In other words, there was the problem of a low figure of merit Z.
In view of the foregoing, it is an object of the present invention to provide a thermoelectric semiconductor material exhibiting satisfactory strength and performance and of high manufacturing yield.